Brass is a widely used material of construction for many articles of commerce. Parts formed from brass generally require a clear coat finish to enhance their luster and appearance and achieve protection against wear or the environment. Solvent-borne liquid coatings have been tried for such purposes, but they fail to deliver the needed performance. Solvent-borne coatings also contain alarmingly high levels of hazardous volatile organic compounds which tend to flash away during curing, making it necessary to contain and collect the vapor of volatile ingredients which is rather costly.
Fusion bonded, thermosetting powder coating compositions have also been used to coat brass parts. Powder coatings offer a number of advantages over liquid coatings. For instance, corrosion and scratch resistance is much superior to that of liquid coatings. In addition, powder coatings are virtually free of the harmful fugitive organic solvents normally present in liquid coatings and, accordingly, give off little, if any, volatiles during curing, which eliminates solvent emission problems and dangers to the health of workers employed in coating operations.
Because brass parts, for example, brass plated zinc die cast parts, are susceptible to outgassing upon heating, thermosetting powder coatings that are capable of curing at low temperatures, e.g., below about 350.degree. F., are generally preferred to minimize substrate outgassing during curing and permanent scarring of the finish coating. Low cure temperatures are also desired, since brass tends to discolor or tarnish at higher temperatures.
Among the commercially available low temperature cure thermosetting powder coatings, GMA acrylic powder coatings have been the most widely used by the brass finishing industry. GMA acrylics offer coatings with exceptional smoothness and clarity, but also suffer from a number of drawbacks including poor adhesion and rather high cost which is becoming increasingly difficult for the brass finishing industry to bear. Attempts have been made to replace the GMA acrylics with traditional thermosetting epoxy powder coatings, such as those based on standard non-crystalline epoxy resins, e.g., bisphenol A type epoxy resins, standard curing agents for epoxy resins, e.g., dicyanodiamide, along with standard catalysts, e.g., 2-methyl imidazole. While epoxies offer improved adhesion and reduced cost, curing of these coatings at the desired low temperature cure conditions, generally leads to bubble entrapment within the finish coating, which is particularly troublesome from an appearance and film quality standpoint.
Bubble entrapment is believed to occur during powder application. In particular, as the powder coating is applied to the substrate, it is believed that air is entrapped within the powder particles as they are deposited on the substrate. When the powder begins to melt, flow and eventually cure, the air must escape from the coating before the finish coating hardens or it will be trapped as tiny bubbles dispersed throughout the coating. With traditional thermosetting epoxy powder coatings, the latter effect tends to occur at the desired low temperature cure conditions, which is believed to be due, at least in part, to the rather high melt viscosities experienced at such tempartures. In a clear coating, those bubbles are especially problematic in that they tend to create an unwanted hazy appearance that interferes with the distinctness of image of the finish coating, i.e., the sharpness of image reflected by a coating's surface. Brass coatings, however, should have a high and visually consistent distinctness of image to enable one to see through the finish coating as if looking at a polished brass part.
What is needed is a thermosetting epoxy powder coating composition adapted to cure at low temperatures below about 350.degree. F. as well as prevent bubble entrapment within the finish coating during heat curing at said low temperatures.